The following patch implements the Xen block backend driver forupstream Linux. This is the host side counterpart to the frontend driverin drivers/block/xen-blkfront.c. The PV protocol is also implemented byfrontend drivers in other OSes too, such as the BSDs and even Windows.

This driver has a long history as an out of tree driver but I amsubmitting it here as a single monolithic patch to aid review. Once ithas been reviewed and is considered suitable for merging can we perhapsconsider merging the equivalent git branch which maintains much ofhistory?

The patch is based on the driver from the xen.git pvops kernel tree buthas been put through the checkpatch.pl wringer plus several manualcleanup passes. It has also been moved from drivers/xen/blkback todrivers/block/xen-blback.